In the manufacture of semiconductor devices, photoresists are used as masks, and after the masking function is completed, it is necessary to remove the photoresist from the semiconductor water. One type of device for stripping such photoresists is known as a plasma afterglow asher. In such a device, a gas such as oxygen which travels through a tube which runs through a microwave enclosure is exalted to a plasma state by microwave energy. The tube exits from the microwave enclosure through an opening, and travels to the region of the semiconductor wafer which is to be stripped. The idea of the device is to effect the stripping with excited atomic species which are present in the tube as extended out of the microwave enclosure (hence the name "afterglow"), rather than with the excited ionic species which are present within the part of the tube which is within the microwave enclosure, and in which the plasma is generated.
However, it has been found in prior art plasma afterglow ashers that microwave energy leaks out of the opening through which the plasma tube exits the microwave enclosure, causing multiple problems, as follows:
a) Energetic ions are produced in the vicinity of the wafer, which can cause wafer damage. The microwave leakage creates plasma in the part of the tube right outside the microwave enclosure, which plasma is a conductor, which causes further microwave leakage to extend further down the tube. This condition leads to ion acceleration by the electric fields which are now present closer to the wafer and produce energetic ions which cause wafer damage.
b) Due to electromagnetic interference with surrounding instruments, the microwave circuit becomes extremely unstable, thus reducing system reliability and limiting process capability.
c) High power applications are impeded by the microwave leakage, stray plasma, and the problems which are associated with system instability.
d) The ultraviolet radiation which is emitted by the stray plasma is also known to cause wafer damage.